Anchor head for post-stressed reinforced concrete structures

ABSTRACT

An anchor assembly for use with the post-stressed strands of a cable-reinforced concrete structure, in which a frusto-conical anchor head seats in the passage of a bearing plate having a frusto-conical central passage, the strands being arranged in the anchor head to have a coaxial resultant tension. A template for use in reseating the grips of the strands during post-stressing is laterally slotted to receive the strands but not the grips and has wings which project through longitudinal recesses in a jack stool. Another template is slotted to receive recessed grips and has removable bars to retain the grips in the slots. A further template has bossed apertures to register with corresponding countersunk passages in an auxiliary anchor head. A jacking device for use in post-stressing the strands comprises an auxiliary anchor head coupled with bearing means located beyond a stool, with a jack interacting between the stool and the bearing means and the auxiliary anchor head being movable along the stool.

United States Patent [191 Shorter 1 ANCHOR HEAD FOR POST-STRESSED REINFORCED CONCRETE STRUCTURES 22 Filed: Aug. 9, 1971 [2]] Appl. No.: 169,996

[30] Foreign Application Priority Data Aug. 19, 1970 Canada 91071 [52] US. Cl 254/29 A [51] Int. Cl E2lb 19/00 [58] Field of Search 254/29 A; 29/452 [5 6] References Cited UNITED STATES PATENTS 3,588,045 6/1971 Stubbs 254/29 A 2,761,649 9/1956 Woolcock.. 254/29 A 3,658,296 4/1972 Yegge 254/29 1,083,353 1/1914 Edison 76/101 SM 3,405,490 10/1968 La Marr 254/29 A [4 1 Apr. 2, 1974 Primary Examiner-Al Lawrence Smith Assistant Examiner-Robert G. Watson [57] ABSTRACT sage of a bearing plate having a frusto-conical central passage, the strands being arranged in the anchor head to have a coaxial resultant tension. A template for use in reseating the grips of the strands during poststressing is laterally slotted to receive the strands but not the grips and has wings which project through 1ongitudinal recesses in a jack stool. Another template is slotted to receive recessed grips and has removable bars to retain the grips in the slots. A further template has bossed apertures to register with corresponding countersunk passages in an auxiliary anchor head. A jacking device for use in post-stressing the strands comprises an auxiliary anchor head coupled with bearing means located beyond a stool, with a jack interacting between the stool and the bearing means and the auxiliary anchor head being movable along the stool.

14 Claims, 9 Drawing Figures PATENTEU APR 21974 SHEET 1 [If 3 NVliN'IUR ROBIN A. C. SHORTER PMENIED APR 2 I974 SNEU 2 BF 3 INVENTOR. ROBIN A.c. SHORTER HY War;

ANCHOR HEAD FOR POST-STRESSED REINFORCED CONCRETE STRUCTURES The present invention relates to reinforced concrete construction and more particularly to the posttensioning of multiple strand cables used in the reinforcement of concrete slabs, beams or columns.

In the post-tensioning of reinforced concrete structures a plurality of parallel, loose strands, forming a cable, extend through an embedded conduit or sleeve in the structure and are tensioned at one or both ends of the structure and anchored. One type of anchorage consists of a cylindrical main anchor head having individual apertures or passages to accommodate each separate strand. The anchor head rests against a bearing plate embedded in the face of the concrete structure. Each strand is held in the anchor head by a tapered and segmented grip resting in a flared portion of the aperture. The strands are tensioned as a group by a hydraulic centre-holed jack bearing against the anchor or against a bearing cup or stool which sits over the anchor head with the strands passing through the cup, the jack, and an auxiliary anchor head which is similar to the main anchor head and which bears against the piston of the jack remote from the bearing cup or is threaded onto a pull rod adjacent the anchor head. The stressing of the strands is effected by extending the piston of the jack, reseating the grips in the auxiliary anchor head, retracting the piston of the jack, and repeating these steps until the required stress in the strands has been achieved. The jack and cup are then removed, the cable is grouted in the conduit, the strands projecting from the anchor head are cut off, and the face of the structure is capped with concrete about the anchor head.

In carrying out the above-mentioned operation, it is necessary to center the anchor head carefully on the bearing plate in order to prevent shearing of the strands. Also, the fabrication of the anchor head requires additional material to provide the necessary overlap with the bearing plate. To overcome these disadvantages it is an object of the present invention to provide an anchor head which seats in the bearing plate and is self-centering.

Various devices have been proposed for reseating the grips in the anchor head each time the strands are stressed by the jack assembly, such as by using springs or a drift wedge. It is another object of the present invention to provide a simplified template for reseating the anchor grips in the anchor head.

The threading of each strand of the cable through the auxiliary anchor head is laborious, including the placing of the grips in the auxiliary anchor head. It is another object of the invention to provide an auxiliary template and an auxiliary anchor assembly for threading the cable strands and seating the grips therein.

Specially constructed centre-hole jacks are used in the post-stressing of the cables in a reinforced concrete structure and these are expensive to manufacture. It is another object of the present invention to provide a jacking assembly in which a conventional jack may be employed.

Example embodiments of the invention are shown in the accompanying drawings in which:

FIG. 1 is an exploded perspective view of one embodiment of an anchor assembly and a centre-hole jack assembly, in association with a multi-strand reinforced cable;

FIG. 2 is a fragmentary view in perspective of the reseating template and bearing stool of FIG. 1;

FIG. 3 is an end view of the auxiliary template of FIG. 1;

FIG. 4 is a plan view, partly in section, of the assemblies of FIG. 1 in operating relationship;

FIG. 5 is a fragmentary view in cross-section of a spreader plate used in association with the auxiliary anchor head assembly of FIG. 4;

FIG. 6 is a planview, partly in section, showing a modified stool and jack for use in association with the anchor assembly of FIG. 1;

FIG. 7 is an end view of the jack shown in FIG. 6;

FIG. 8 isa cross-sectional view taken along the line 88 of FIG. 6; and

FIG. 9 is a plan view of a modified form of the stool and jack shown in FIG. 6.

The embodiment of the invention in FIGS. 1 to 4 of the drawings shows a concrete slab or beam 10 reinforced with a plurality of cable strands 11 passing through a conduit (not shown) embedded in the beam and emerging from the conduit through a trumpet 12. An annular bearing plate 13 is embedded in the face of beam 11 and is fixed to the flared end of trumpet 12. A frusto-conical passage 14 in bearing plate 13 flares outwardly to form a continuation of the passage of trumpet 12 and terminates in a face 15 exposed in a recessed 10a of slab 10. An annular flange 16 forms a base for bearing plate 13 with spaced reinforcing fins 17 being radially located around the plate.

An anchor head 20, of frusto-conical shape, registers with bearing plate 13 in passage 14 (as seen in FIG. 4 of the drawings). The frusto-conical side surface of the head is preferably inclined to the axis of the head at an angle between 7 and 10. A plurality of passages 21 traverse anchor head longitudinally and these passages each flare outwardly adjacent larger face 22 of the head to form sockets 23 to accommodate segmented, tapered grips 24 of conventional construction.

An hydraulic jack assembly, used to tension strands l l of the reinforcing cable, comprises a centre-hole hydraulic jack having a longitudinal bore 31 which is axially co-extensive with the longitudinal-axis of an annular bearing stool 32 circumscribing anchor head 20 and resting against face 15 of bearing plate 13. A shoulder 32a on bearing stool 32 overhangs the periphery of face 22 of anchor head 20. A pair of recesses 33 on opposite sides of the base portion of stool 32 accommodate opposed wings 34 extending outwardly from a circular finger template 35 which has parallel, transverse slots 36 oriented and of a widthto pass'strands 11 but not grips 24. Recesses 33 of stool 32 are of a depth allowing some movement of template 35 away from grips 24 and face 22 of anchor head 20 but not enough of such movement to allow the grips to drop out of their seats in the anchor head.

As seen in FIG. 2 of the drawings, a rod 37 may impinge on each wing 34 of template 35, one end of the rod being threaded into a bore 37a and the other end seating loosely in a recess 37b in the wing, with a knurled head 37c integral with the rod and located on the rod between the bore and the recess. A circumscribing raised shoulder 39 on the end face of stool 32 centres one end of jack 30 against the stool.

An ,annular piston 38 projects from the other end of the jack. An auxiliary anchor head 40 is adapted to seat on the free end of piston 38 and is centered on the piston by a shoulder 40a on the auxiliary anchor head, which also has passages 41 for passing strands 11. Passages 41 flare towards the face of head 40 opposite shoulder 40a to form sockets seating tapered and segmented grips 42 which differ from grips 24 in each having a circumferential recess forming a thin shank 43 at the flared end of the grip terminating in a flange 44 (see FIG. of the drawings). An auxiliary template 45 is adapted to engage grips 42, having a plurality of lateral slots 46 (similar to slots 36 in template 35) oriented with respect to the grips to accommodate shanks 43 of the grips, which are retained loosely on the auxiliary template by flanges 44 of the grips. A plurality of retaining bars 47 are receivable in passages 48 passing transversely through template 45 to locate the bars ad jacent shank 43 on that side of each shank whereby grips 42 are prevented from slipping out from slots 46. Template 45 may be secured to'auxiliary anchor head 40'by bolts 49.

A locating template 50, having apertures 51 to pass strands 11, is slidable along the strands within stool 32 and bore 31 ofjack 30. One face of template 50 carries bosses 52, preferably hemispherical, which circumscribe apertures 51, and these bosses are adapted to register with recesses 53 circumscribing passages 41 in the face of head 40 carrying shoulder 40a. A spreader plate 55 shown in FIG. 5 of the drawings may be employed to spread grips 42 for the reception of strands 11. Plate 55 carries on one face a plurality of pins 56 located to register with grips 42 held by template 45, with the ends of the pins bearing against teeth 57 of the grips beyond the plane of the template, thus pivoting the segments of each grip about shank 43 to widen the passage at the narrow end of the grip, as indicated by arrow 58 in FIG. 5.

In the operation of the embodiment shown in FIGS. 1 to 5 of the drawings, strands 11 leading through trumpct 12 and passage 14 of bearing plate 13 are threaded through passages 21 in main anchor head 20 and grips 24 are seated in sockets 23 about the strands. Template 35 is then passed laterally onto strands 11 and located adjacent grips 24. When template 35 is in place, bearing stool 32 is set against face of bearing plate 13, with wings 34 of the template projecting through recesses 33 in the stool and rods 37 seating in recesses 37b. Jack 30 is then placed against stool 32, with-strands 11 being passed through both the stool and the jack and then being threaded through template 50 (or strands 11 may first be threaded through template 50 when anchor head is being threaded, with stool 32 and jack being slipped over the strands thus held by the template).

Auxiliary anchor head 40, grips 42 and auxiliary template are next assembled as shown in FIG. 3 of the drawings and the auxiliary template is then connected to the auxiliary anchor head by bolts 49, with grips 42 loosely seated in the sockets of passages 41 of the auxiliary head. By locating template close to the ends of strands l1, and by inserting pins 56 of plate into grips 42 to spread the grips, the strands are easily threaded into auxiliary anchor head 40 by bringing the auxiliary anchor head up against template 50 to register bosses 52 in recesses 53 of the auxiliary anchor head,

5 jack 30, which unseats grips 24 from main anchor head 20 and seats grips 42 in auxiliary anchor head 40. To reseat grips 24 in sockets 23 in anchor head 20, template 35 is forced against the grips by unscrewing rods 37 by means of heads 370 or alternately, by applying pressure externally to wings 34 projecting from stool 32. Piston 38 is then retracted into jack 30 and auxiliary anchor head 40 is reseated on the piston, with grips 32 moving in conjunction with theauxiliary anchor head because of the connection of auxiliary template 45 with the auxiliary anchor head. This stressing step is repeated until strands 11 have been tensioned to the required amount, after which the jacking assembly is removed and the excess of strands 11 projecting from main anchor head 20 are severed.

It will be appreciated that use of anchor head 20 shown in FIGS. 1 and 4 of the drawings brings bearing plate 13 closer to the face of slab 10, which shifts the bearing location of the load closer to the slab face.

In the embodiment of the invention shown in FIG. 6 to 8 of the drawings, an hydraulic jack 60 having a solid piston 61 is used with bearing plate 13, anchor head 20 and template 35 of the previous embodiment shown in FIGS. 1, 2 and 4. Cylinder 62 of jack 60 carries a transverse yoke 63 having a pair of tie rods 64 each bolted at one end to the yoke and at the other end to an auxiliary anchor head 65 spaced by the rods from jack 60. Either or both yoke 63 and anchor head 65 may be laterally slotted to receive rods 64. Rods 64 straddle a rectangular stool 66 having four legs 67 depending on a seat 68 with the legs being reinforced on opposing sides of the stool by crossbars 69. Legs 67 rest against face 15 of bearing plate 13 while piston 61 of jack 60 bears against seat 68 of the stool. Auxiliary anchor head 65 carries a plurality of passages 71 to pass strands l1 and each passage flares to seat segmented grips 72 associated with the strands. Tie rods 64 lie astride stool 66 on the two opposing sides of the 'stool not carrying cross bars 69, which allows auxiliary anchor head 65 to be moved towards seat 68 of the stool. Wings 34 of slotted template 35 extend outwardly from each side of stool 66 below a pair of restraining bars 80, located below crossbars 69, which permit only restricted movement of template 35 away from face 22 of main anchor head 20. If desired, rods 37 may be used as in the previous embodiment. A further pair of crossbars 81 at the foot of stool 66 below tie rods 64 provide a wider bearing surface and added reinforcement for the stool.

In the operation of the embodiment shown in FIGS. 6 to 8 of the drawings, strands ll emerging from trumpet l2- and passage 14 of bearing plate 13 are threaded through anchor head 20, grips 24 are seated in passages 23 of the anchor head about the strands, and template 35 is located on the strands adjacent the grips, as before. Strands 11 are next threaded through auxiliary anchor head 65 with grips 72 seated in the auxiliary anchor head about the strands. Grips 72 in auxiliary anchor head 65 may be retained therein by the use of template 45 with modified grips 72 shown in FIGS. 1

to 5 of the drawings. Stool 66 is then placed over auxiliary anchor head 65 and template 35 with legs 67 bearing against face 15 of anchor plate 13 and with the auxiliary anchor head projecting from the sides of the stool not carrying Crossbars 69. Wings 34 project from stool 66 beneath restraining bars 80 and in a direction normal to the direction of projection of the auxiliary anchor head from the stool. Jack 60 is then placed against stool 66 with piston 61 bearing against seat 68, and tie rods 64 are bolted both to yoke 63 and to auxiliary anchor head 65.

Hydraulic jack 60is actuated to extend piston 61 which bears against seat 68 of stool 66 and pulls auxiliary anchor head 65 away from anchor head 20. This ac tion unseats grips 24, which are retained loosely in flared passages 71 by restraining bars 80 acting on wings 34 of template 35, and the grips may be reseated by applying suitable pressure externally on the wings of the template. Piston 61 is then retracted into cylinder 62 ofjack 60 to move auxiliary anchor head 65 towardsmain anchor head 20. When grips 72 have been reseated in auxiliary anchor head 65, strands 11 are again stressed and the operation is repeated until the required tension has been imparted to the strands.

A variation in the structure ofjack 60 shown in FIGS. 6 to 8 of the drawings is illustrated in FIG. 9 of the drawings in which ajack 90 has a solid piston 91 bearing against a transverse plate 92 connected by a pair of tie rods 93 (straddling stool 66) to auxiliary anchor head 65. Cylinder 94 ofjack 90 rests against seat 68 of stool 66 while piston 91 bears against transverse plate 92. The operation of this embodiment is the same as the previous embodiment except that piston 91 bears against transverse plate 92 and moves the transverse plate away from bearing plate 13 while the cylinder does not move in relation to stool 66. With this construction and operation, the hydraulic jack does not have to be adapted to carry a yoke fixed on the cylinder as in the previous embodiment.

It will be appreciated that stool 66 can be constructed in any suitable transverse shape which provides a pair of opposed slots allowing auxiliary anchor head 65 to move longitudinally within the stool and recesses to accommodate wings 34 of template 35 if that template is to be used with the jacking assembly. This construction allows access to the face of anchor head 20, which for instance enables strand slippage to be measured accurately.

The use of the embodiment shown in FIGS. 6 to 9 of the drawings provides a saving in strand lengths and also allows the use of a conventional jack which is less expensive to produce and easier to handle.

I claim:

1. An anchor assembly for use with the post-stressed strands of a cable-reinforced concrete structure, comprising:

a bearing plate having a frusto-conical central passage therethrough tapering inwardly from its bearing face, and

a frusto-conical anchor head adapted to seat in the central passage of the bearing plate, a plurality of passages traversing the anchor head longitudinally and each adapted to pass one of the strands, each of the passages terminating in a tapered socket in the larger face of the anchor head for seating a tapered cable grip therein, the passages being located in a balanced pattern with respect to the central 6 longitudinal axis of the anchor head whereby the resultant tension of the strands when secured in the anchor head passes through the area of said longitudinal axis and through the area of the central longitudinal axis of the central passage of the bearing plate when the anchor head is seated therein.

2. An anchor assembly as claimed in claim 1 in which the taper of the frusto-conical wall of the central passage of the bearing plate and the taper of the frustoconical wall of the anchor head is between 7 and 10 with respect to the central longitudinal axis thereof.

3. An anchor assembly as claimed in claim 1 in which the bearing plate has a circumscribing, laterally outstanding flange adjacent the face of the flange opposite the bearing face.

4. A device for use with a jack in post-stressing the strands of a cable-reinforced concrete structure emanating from an anchor head seating tapered grips and associated with a bearing plate, comprising in combination:

a stool adapted to rest against the bearing plate about the anchor head and to transmit pressure from the jack to the bearing plate, the stool having opposed recesses extending along the sides from the base thereof; and i a template laterally slotted to receive the strands whereby the template is located to face the anchor head and the anchor grips, the template having opposed wings in the plane thereof adapted to project through the recesses of the stool, the recesses being of a length along the stool to allow movement of the template away from the anchor head, upon tensioning of the strands to cause the grips in the anchor head to bear against the template, a distance sufficient to allow the grips in the anchor head to be unseated by the strands while retaining the grips loosely in position about the strands.

S. A device as claimed in claim 4 including means located between the stool and the wings of the template and adapted on actuation to bear against both the stool and the wings to move the template towards the anchor head.

6. A device for use in post-stressing the strands emanating from an anchor head of a cable-reinforced structure comprising;

a plurality of tapered, segmented grips adapted to seat in the anchor head, each grip having an annular recess adjacent the wide end thereof to form a shank of reduced diameterand a circumscribing end flange;

a template laterally slotted to receive loosely the shanks of the grips when the grips are arranged to seat in the anchor head, and bars removably located in transverse passages in the plane of the template to retain the grips loosely in the slots.

7. A device as claimed in claim 6 including a spreader plate carrying a plurality of pins located to register with the bores of the grips whereby on inserting the pins into the heads of the grips the opposite ends of the grip segments are spread apart to facilitate passage of the strands into the grips. I

8. A device for use with a jack in post-stressing the strands of a cable-reinforced concrete structure emanating from a main anchor head associated with a bearing plate, comprising in combination:

a stool having a seat and a weight bearing member depending from the seat to rest against 'the bearing plate about the main anchor head;

an auxiliary anchor head located between the seat and the main anchor head and coaxial therewith;

bearing means located on that side of the seat of the stool remote from the weight bearing member and connected with the auxiliary anchor head; and

jack means interacting between the bearing means and the seat to move the bearing means away from the seat whereby the auxiliary anchor head is moved away from the main anchor head.

9. A device as claimed in claim 8 in which the bearing means comprises a plate, the plate being interconnected with the auxiliary anchor head by meansstraddling the seat, and the jack means being positioned between the seat and the plate to move the plate away from the seat.

10. A device as claimed in claim 8 in which the auxiliar'y anchor head projects laterally from longitudinal slots in the stool on each side thereof, and including tie rods interconnecting the laterally projecting portions of the auxiliary anchor head and the bearing means.

11. A device as claimed in claim 8 in which the weight bearing member comprises four legs extending from the seat, opposite pairs of said legs carrying interconnecting reinforcing bars.

12. A device as claimed in claim 8 including a template laterally slotted to receive the strands and locate the template facing the anchor head, the template being circumferentially dimensioned to be moved along the strands within the stool and having a pair of 8 opposed wings in the plane of the template of a length to project laterally through recesses longitudinally disposed in the base of the stool.

13. A device as claimed in claim 8 in which the jacking means comprises an hydraulic jack having'a cylinder member and a piston member and the bearing means comprises a yoke inter gral with one of said jack members and extending laterally thereof, the yoke being interconnected with the auxiliary anchor head, the jack being oriented to bear the other of said jack members against the seat of the stool.

14. In a device for post-stressing the strands of a cable-reinforced structure using an anchor head and an auxiliary anchor head separated by a centre hole jack, each of the anchor heads having a separate passage therethrough to pass each strand, a locating template having a separate aperture therethrough to pass each strand, the apertures in the template being alignable with the passages in the anchor heads and having a diameter less than the bore diameter of the jack whereby the recesses in the auxiliary anchor head. 

1. An anchor assembly for use with the post-stressed strands of a cable-reinforced concrete structure, comprising: a bearing plate having a frusto-conical central passage therethrough tapering inwardly from its bearing face, and a frusto-conical anchor head adapted to seat in the central passage of the bearing plate, a plurality of passages traversing the anchor head longitudinally and each adapted to pass one of the strands, each of the passages terminating in a tapered socket in the larger face of the anchor head for seating a tapered cable grip therein, the passages being located in a balanced pattern with respect to the central longitudinal axis of the anchor head whereby the resultant tension of the strands when secured in the anchor head passes through the area of said longitudinal axis and through the area of the central longitudinal axis of the central passage of the bearing plate when the anchor head is seated therein.
 2. An anchor assembly as claimed in claim 1 in which the taper of the frusto-conical wall of the central passage of the bearing plate and the taper of the frusto-conical wall of the anchor head is between 7* and 10* with respect to the central longitudinal axis thereof.
 3. An anchor assembly as claimed in claim 1 in which the bearing plate has a circumscribing, laterally outstanding flange adjacent the face of the flange opposite the bearing face.
 4. A device for use with a jack in post-stressing the strands of a cable-reinforced concrete structure emanating from an anchor head seating tapered grips and associated with a bearing plate, comprising in combination: a stool adapted to rest against the bearing plate about the anchor head and to transmit pressure from the jack to the bearing plate, the stool having opposed recesses extending along the sides from the base thereof; and a template laterally slotted to receive the strands whereby the template is located to face the anchor head and the anchor grips, the template having opposed wings in the plane thereof adapted to project through the recesses of the stool, the recesses being of a length along the stool to allow movement of the template away from the Anchor head, upon tensioning of the strands to cause the grips in the anchor head to bear against the template, a distance sufficient to allow the grips in the anchor head to be unseated by the strands while retaining the grips loosely in position about the strands.
 5. A device as claimed in claim 4 including means located between the stool and the wings of the template and adapted on actuation to bear against both the stool and the wings to move the template towards the anchor head.
 6. A device for use in post-stressing the strands emanating from an anchor head of a cable-reinforced structure comprising; a plurality of tapered, segmented grips adapted to seat in the anchor head, each grip having an annular recess adjacent the wide end thereof to form a shank of reduced diameter and a circumscribing end flange; a template laterally slotted to receive loosely the shanks of the grips when the grips are arranged to seat in the anchor head, and bars removably located in transverse passages in the plane of the template to retain the grips loosely in the slots.
 7. A device as claimed in claim 6 including a spreader plate carrying a plurality of pins located to register with the bores of the grips whereby on inserting the pins into the heads of the grips the opposite ends of the grip segments are spread apart to facilitate passage of the strands into the grips.
 8. A device for use with a jack in post-stressing the strands of a cable-reinforced concrete structure emanating from a main anchor head associated with a bearing plate, comprising in combination: a stool having a seat and a weight bearing member depending from the seat to rest against the bearing plate about the main anchor head; an auxiliary anchor head located between the seat and the main anchor head and coaxial therewith; bearing means located on that side of the seat of the stool remote from the weight bearing member and connected with the auxiliary anchor head; and jack means interacting between the bearing means and the seat to move the bearing means away from the seat whereby the auxiliary anchor head is moved away from the main anchor head.
 9. A device as claimed in claim 8 in which the bearing means comprises a plate, the plate being interconnected with the auxiliary anchor head by means straddling the seat, and the jack means being positioned between the seat and the plate to move the plate away from the seat.
 10. A device as claimed in claim 8 in which the auxiliary anchor head projects laterally from longitudinal slots in the stool on each side thereof, and including tie rods interconnecting the laterally projecting portions of the auxiliary anchor head and the bearing means.
 11. A device as claimed in claim 8 in which the weight bearing member comprises four legs extending from the seat, opposite pairs of said legs carrying interconnecting reinforcing bars.
 12. A device as claimed in claim 8 including a template laterally slotted to receive the strands and locate the template facing the anchor head, the template being circumferentially dimensioned to be moved along the strands within the stool and having a pair of opposed wings in the plane of the template of a length to project laterally through recesses longitudinally disposed in the base of the stool.
 13. A device as claimed in claim 8 in which the jacking means comprises an hydraulic jack having a cylinder member and a piston member and the bearing means comprises a yoke intergral with one of said jack members and extending laterally thereof, the yoke being interconnected with the auxiliary anchor head, the jack being oriented to bear the other of said jack members against the seat of the stool.
 14. In a device for post-stressing the strands of a cable-reinforced structure using an anchor head and an auxiliary anchor head separated by a centre hole jack, each of the anchor heads having a separate passage therethrough to pass each strand, a locating template having a separate aperture therethrough To pass each strand, the apertures in the template being alignable with the passages in the anchor heads and having a diameter less than the bore diameter of the jack whereby the locating template is slidable through the jack bore, a boss circumscribing each aperture on one side of the template, a recess in the auxiliary anchor head circumscribing each passage at the end thereof facing the jack bore, the bosses on the template being registrable with the recesses in the auxiliary anchor head. 